1. Field of the Invention
The present invention relates to the field of turbomachines such as an axial compressor of a gas turbine engine, and is particularly intended for the variable-pitch stator blades of the machine.
1. Description of the Related Art
An articulated system, such as the variable-pitch stator blades of a gas turbine engine compressor, comprises parts that move relative to one another. FIGS. 1 and 2 show schematically a variable-pitch stator blade 1 mounted in the casing 3 of the machine. The stator blade comprises an aerofoil 12, a plate or platform 13 and a rod forming a pivot 14 at one end. The pivot 14 is housed in a bore or radial orifice made in the wall of the casing 3 via various bearings. The blade is held by this end only. The other end holds an annular floating element 16 in which it is mounted so as to pivot via a second pivot 17. The ring is provided with sealing means for the portion of the rotor 18 that is adjacent to it. The pivot 14 swivels in the corresponding bore of the casing by means of bearings, for example a bottom bearing 4. The platform 13 is housed in a cavity in the form of a counterbore machined in the wall of this casing. The wall of the casing is in radial contact with the platform 13 either directly or by means of a bush or shim. The top portion of the pivot 14 is held in a top bearing 5. The opposite face of the platform 13 relative to the bearing 4 forms the base of the aerofoil and is swept by the gases set in motion by the compressor. This face of the plate is shaped so as to ensure the continuity of the stream formed by the casing. A nut holds the blade in its housing and a lever actuated by appropriate control members controls the rotation of the blade about the axis XX of the rod in order to place the latter in the required position relative to the line of the gaseous flow. The relative movements result from the sliding of the surfaces in contact with one another.
In the case of an axial compressor of a gas turbine engine or else an axial compressor only of air or another gas, such as a blast furnace or natural gas, the aerofoil 12 is subjected over the whole of its length to the aerodynamic and pressure forces generated by the gaseous flow. The component of these forces oriented perpendicularly to the chord in the pressure side to suction side direction, usually passing via the axis of the pivot, is the greatest. It is noted however that, in the case of major deflections, the component may move away from this axis. The aerofoil is also subjected to axial forces of static pressure directed upstream because of the pressure difference between downstream and upstream. The resultant force is illustrated by the arrow F in the figures. The result of this is the application of a moment that, associated with the rotation of pitch about the axis XX over an amplitude that may reach and exceed 40 degrees, creates an intense zone of friction. This friction leads secondarily to wear of the plate and/or the bushes. This first zone 20 of intense friction is located on a portion of the surface of the plate. It is indicated by crosses in FIG. 2. Therefore, in normal operation of the machine, because of these tilting forces applied to the aerofoil 12, the plate presses via this first zone 20 against the surface of the housing made in the wall of the casing, while on the portion diametrically opposed to the pivot, the pressing forces are zero or very slight.
In the aeronautical field, any excess weight should be avoided and independently of the excess pressure of any excess load, there is also an attempt to eliminate any weight that fulfils no function whether it be mechanical or aerodynamic.
The applicant also has the constant objective of finding solutions that make it possible to lighten the machine without, for all that, compromising its performance and its reliability. Any weight saving improves the efficiency of the machine and makes it possible to reduce operating costs.